libdw: Add new dwarf_cu_info function.

libdw: Add new dwarf_cu_info function.

This allows getting a (split) subdie lazily, only when needed.
All arguments to dwarf_get_units are optional. When not given
then unit DIE and sub DIE are not looked up. This new function
allows them to be looked up when not immediately retrieved with
dwarf_get_units, or for a Dwarf_CU gotten through some other way.

Add a new testcase to make sure the results of calling dwarf_cu_info
and dwarf_get_units are consistent.

Signed-off-by: Mark Wielaard <mark@klomp.org>